Method of making a permanent magnet



Sept 1967 F. TOMHOLT ETAL v METHOD OF MAKING A PERMANENT MAGNET FiledMarch 28, 1963 INVENTOR FRITS TOMHOLT BY WILLEM EISE WITTEVEEN W AGENTUnited States Patent 3,341,940 METHOD OF MAKING A PERMANENT MAGNET FritsTomholt and Willem Eise Witteveen, Emmasingel, Eindhoven, Netherlands,assignors to North American Philips Company, Inc., New York, N.Y., acorporation of Delaware Filed Mar. 28, 1963, Ser. No. 268,782 Claimspriority, applicatizo7rzNgtherlands, Mar. 29, 1962,

4 Claims. (51. 29-608) ABSTRACT DISCLOSURE Our invention relates to amethod of manufacturing a magnetically anisotropic body and to amagnetically anisotropic body manufactured by this method.

Magnetically anisotropic magnets have been made by compressing a finelydivided permanent magnetic material .in a magnetic field which orientsthe magnetic material, after which the compressed oriented material issintered into a highly coherent body. Naturally, it is desirable in thiscase that the magnetic orientation produces a maximum effect, so thatthe compressed mass is indeed strongly magnetically anisotropic.However, it has been found that the particles which should be orientedmutually in parallel with their easy direction of magnetization show atendency to conglomerate because they attract each other magneticallywhile forming magnetically isotropic conglomerates, as a result of whichthe effect of the magnetic orientation is decreased.

It is a principal object of our invention to minimize the tendency ofthe particles to form isotropic conglomerates during orientation.

A further object of our invention is to provide a meth- 0d ofmanufacturing anisotropically permanent magnets having improvedanisotropic properties. These and further objects of the invention willappear as the specification progresses.

According to the invention, We completely disintegrate the material fromwhich a permanent magnet is to be made, in a closed receptacle, to apowder having an average particle size under 1 micron. This powder isthen whirled or centrifuged to form a cloud of dust from which thepowder particles are separated under the influence of a magnetic fieldproduced within the closed receptacle and united to form magneticallyanisotropic threads, chains or skeins hereinafter referred to asmagnetically anisotropic conglomerates. The latter, after removal fromthe closed receptacle, are compressed into a body which then is sinteredto form a highly-coherent body.

These magnetically anisotropic threads, chains or skeins areunexpectedly quite resistant to the disorienting influence of externalmechanical forces which are exerted on them when they are transferred toa die; in fact, formation of magnetically isotropic conglomeratesvirtually does not take place.

The invention is of particular importance for the manufacture of ceramicanisotropic magnets having as the constituent essential for thepermanent magnetic properties at least one material, in the form ofhexagonal crystals, with a chemical composition according to the formulaM0.6Fe- O where M is at least one of the metals Ba (barium), Sr(strontium) and Pb (lead). The proportions of the Weiss domains of thesaid crystals generally lie within the range of approximately 1 to 3microns, so that for an average particle size under 1 micron, forexample approximately 0.3 micron, by far most of the particles each aresingle Weiss domains. That is to say, these particles are micro-magnetsmagnetized spontaneously and up to the saturation, which have a strongtendency to conglomerate while forming the above-mentioned magneticallyisotropic conglomerates. However, if it is possible to substantiallyprevent this conglomeration, a maximum magnetic orientation effect canactually be obtained just in this case. The present invention offers ameans which is excellently suited to this purpose.

According to a particular embodiment of the invention, the permanentmagnetic material from which the magnet body to be manufactured isconstituted, is disintegrated and whirled to form a cloud of dust in acontainer which is closed with a cover on the inside of which theabove-mentioned magnetically anisotropic conglomerates, i.e. threads,chains or skeins deposit.

The magnetic field, under the influence of which the magneticallyanisotropic threads, chains or skeins are formed, is usually produced byan electromagnet which is rendered operative after at least a part ofthe permanent magnetic material is disintegrated to an average particlesize under 1 micron and is whirled to form a cloud of dust. Preferably,the electromagnet is provided in the cover of the container.

The invention will now be described with reference to a particularexample which is illustrated only, and the accompanying drawing the solefigure of which shows an apparatus for carrying out the invention.

Finely-divided permanent magnet material 8, e.g. barium hexaferrite,BaO.6Fe O initially ground in a ball mill, is placed in a metalcontainer 1 provided with a metal cover 4 which hermetically seals thecontainer. Cover 4 is provided with a neck 5 in which a soft iron magnetcore 6 is provided Which is wound with wire turns 7, the core and thewire turns together forming an electromagnet. Extending through thebottom of the container is a shaft 2 which supports a set of blades 3for rotation.

After the apparatus has been switched on, the permanent magnetic powderis disintegrated by the metal bands. rotating at high speed, to anaverage particle size of approximately 0.5 micron, and whirled to form acloud of dust. Two minutes later the electromagnet is switched on. Inthe neck of the cover the magnetically anisotropic threads, chains orskeins 9 of powder particles soon deposit in large numbers. Afteranother two minutes grinding is discontinued, the cover is removed fromthe container, its lower side placed over a filling funnel, and theelectromagnet switched off. The magnetically anisotropic chains are nolonger attracted by the cover and fall into the filling funnel. Fromthere the mass is transferred to a die, in which by means of anelectromagnet a substantially homogeneous magnetic field is produced.The mass is then compressed in the magnetic field under a pressure of0.5 ton/cmfi. The compressed body is removed from the die and sinteredby heating it for 24 hours at a maximum temperature of 1290 C. Theresulting magnet body at room temperature has a (BH) of 3.8 l0 gaussoersted and a magnetic anisotropy, expressed in the value of thequotient B ll/B l, of 4.0 (where 13,11 is the maximum remanence measuredin a direction parallel to the principal direction of magnetization andB l the remanence measured in the direction at right angles to the di-J; rection in which the maximum remanence B ll is measured).

While we have described the invention with reference to a particularapparatus and a particular material, it will be obvious to those skilledin the art that other forms of apparatus and other materials could beused equally well. For instance, instead of barium hexaferrite, otherwellknown magnetic materials such as magnetic alloys which can be formedinto sintered bodies may be subjected to a similar treatment. Likewise,the material could be ground initially to the required fine state ofsubdivision and centrifuged by a conventional centrifuge, the magneticfield being applied from an external source.

Therefore, we wish it to be clearly understood that the foregoingexample is illustrative only, the invention itself being limited onlyand defined by the appended claims.

We claim:

1. A method of manufacturing a magnetically anisotropic body comprisingthe steps, disintegrating a finelydivided magnetic material in a closedreceptacle, whirling said material in said receptacle to form a dustcloud of said material, separating particles of said material in saiddust cloud with a magnetic field which forms magnetically anisotropicconglomerates of said particles, removing said conglomerates from saidreceptacle, compressing the magnetically anisotropic conglomerates ofsaid particles in the presence of a magnetic field into a body, andsintering said body to form the same into a highly coherent body.

2. A method of manufacturing a magnetically anisotropic body comprisingthe steps, disintegrating a finelydivided magnetic material having acomposition consisting of MO-6Fe O where M is a metal selected from thegroup consisting of Ba, Sr and Pb in a closed receptacle, whirling saidmaterial in said receptacle to form a dust cloud of said material,separating particles of said material in said dust cloud with a magneticfield which forms magnetically anisotropic conglomerates of saidparticles, removing said conglomerates from said receptacle, compressingthe magnetically anisotropic conglomerates of said particles in thepresence of a magnetic field into a body, and sintering said body toform the same into a highly coherent body.

3. A method of manufacturing a magnetically anisotropic body comprisingthe steps, disintegrating a finelydivided magnetic material consistingessentially of BaO-6Fe O in a closed receptacle, whirling said materialin said receptacle to form a dust cloud of said material, separatingparticles of said material in said dust cloud with a magnetic fieldwhich forms magnetically anisotropic conglomerates of said particles,removing said conglomerates from said receptacle, compressing themagnetically anisotropic conglomerates of said particles into a body inthe presence of a magnetic field to form a magnetically anisotropicbody, and sintering said body to form the same into a highly coherentbody.

4. A method of manufacturing a magnetically anisotropic body comprisingthe steps, disintegrating a finelydivided magnetic material consistingessentially of BaO-6Fe O in a closed receptacle, whirling said materialin said receptacle to form a dust cloud of said material, separatingparticles of said material in said dust cloud with a magnetic fieldwhich forms magnetically anisotropic conglomerates of said particles,removing said conglomerates from said receptacle, compressing themagnetically anisotropic conglomerates of said particles into a body inthe presence of a magnetic field to form a magnetically anisotropicbody, and sintering said body at a temperature of about 1290 C. fortwenty-four hours to form the same into a highly coherent body.

References Cited UNITED STATES PATENTS 2,186,659 1/1940 Vogt 29-155.62,384,215 9/ 1945 Toulmin et al 29-420 2,856,273 10/1958 Beeber et al23--288.3 2,974,887 3/1961 Grandinetti 241-101 3,033,474 5/1962 OToole241l01 3,090,107 5/1963 Minden et a1 29-155.6 3,189,667 6/1965 Buttneret al. 29155.6

CHARLIE T. MOON, Primary Examiner.

C. E. HALL, I. W. BOCK, Assistant Examiners.

1. A METHOD OF MANUFACTURING A MAGNETICALLY ANISOTROPIC BODY COMPRISINGTHE STEPS, DISINTEGRATING A FINELYDIVIDED MAGNETIC MATERIAL IN A CLOSEDRECEPTACLE, WHIRLING SAID MATERIAL IN SAID RECEPTACLE TO FORM A DUSTCLOUD OF SAID MATERIAL, SEPARATING PARTICLES OF SAID MATERIAL IN SAIDDUST CLOUD WITH A MAGNETIC FIELD WHICH FORMS MAGNETICALLY ANISOTROPICCONGLOMERATES OF SAID PARTICLES, REMOVING SAID CONGLOMERATES FROM SAIDRECEPTACLE, COMPRESSING THE MAGNETICALLY ANISOTROPIC CONGLOMERATES OFSAID PARTICLES IN THE PRESENCE OF A MAGNETIC FIELD INT A BODY, ANDSINTERING SAID BODY TO FORM THE SAME INTO A HIGHLY COHERENT BODY.